This invention relates to crystalline aluminosilicates. In particular, it relates to methods of preparing crystalline aluminosilicates and especially improved preparation methods which include the preparation of a crystalline aluminosilicate precursor which may be stored and utilized subsequently for the preparation of crystalline aluminosilicates.
Synthetic crystalline aluminosilicates constitute well known materials which have heretofore been employed as selective absorbents, carriers and catalysts. In general such crystalline materials have been grown under designated conditions of temperature and time from alkali oxide, aluminum oxide, silica and water precursors. The simplest source materials for preparing the more common crystalline aluminosilicates, the type A, X and Y zeolites, are sodium aluminate, sodium silicate and for the more siliceous X and Y types an additional source of SiO.sub.4.sup.-4 ions. (The terms crystalline aluminosilicates and zeolites are used herein interchangably and refer to the same crystalline materials.) Most of the synthesis procedures now in use are tailored to the specific zeolite being prepared. During the course of preparation a wet gel is produced which is often susceptible to nucleation by unwanted species in the preparation mixture. In many instances this obviates the storage of the wet gel. In one such preparation process described and claimed in U.S. Pat. No. 2,847,280, a type 4A zeolite is prepared by forming a mixture of hydrous silica and by-product sodium salt by reacting an aqueous solution of sodium silicate with a substance selected from the group consisting of CO.sub.2, SO.sub.2, H.sub.2 S, the sodium hydrogen salts of their corresponding acids and mixtures of the same; adding sodium aluminate to said mixture in an amount sufficient to establish the proportions of aluminum and silicon in the resultant mixture essentially stoichiometric for the formation of zeolite A; aging the resultant mixture for between 8 and 170 hours at a temperature not substantially above about 100.degree. F. thereafter maintaining the aged mixture under autogenous pressure at a temperature of 150.degree.-325.degree. F. for at least about three hours; and recovering the hydrated sodium aluminosilicate as the resultant solid fraction in crystalline form.
Another process for preparing a 4A zeolite is described and claimed in U.S. Pat. No. 2,882,243. This process comprises preparing an aqueous solution of oxides within specified ranges of their mole ratios, or of materials whose chemical compositions can be completely represented as mixtures of the oxides, Na.sub.2 O, Al.sub.2 O.sub.3, SiO.sub.2, and H.sub.2 O. The mixture is maintained at a temperature of about 100.degree. C. for periods ranging from fifteen minutes to ninety hours or longer to yield the crystallized product, sodium zeolite A.
The composition of the crystalline zeolites of significant interest are set forth below;
______________________________________ TYPE OF ZEOLITE CHEMICAL ANALYSIS ______________________________________ A Na.sub.2 O . Al.sub.2 O.sub.3 . 2SiO.sub.2 . 4.5H.sub.2 O X Na.sub.2 O . Al.sub.2 O.sub.3 . 2.5 SiO.sub.2 . 6H.sub.2 O Y Na.sub.2 O . Al.sub.2 O.sub.3 . 5 SiO.sub.2 . 8H.sub.2 ______________________________________ O
A number of synthesis procedures for the preparation of crystalline aluminosilicates is known. In most instances the process is a continuous one involving a number of steps of varying complexity. In the crystalline aluminosilicate preparations of Barrer and his co-workers, aqueous mixtures of the preparations were evaporated to dryness over a steam table or in an air oven prior to crystalline formation. The dried material was hard and had to be ground to a powder to permit reconstitution. Reconstitution for completion of crystalline formation required the addition of water plus alkali. A new procedure for preparing a number of crystalline aluminosilicates which involves the recovery of an amorphous zeolite precursor in powdered form, said precursor being capable of isolation and storage in a dry, stable form for ultimate use in providing a variety of zeolites and which could be reconstituted merely by the addition of water, would offer significant advantages over processes presently employed.